Surface and bulk-micromachining techniques have been well developed to fabricate various microelectromechanical system (MEMS) and microelectroopticalmechanical system (MEOMS) structures on the silicon substrate, such as actuators, microheaters, accelerometers, detectors, anemometers, optical transducers such as light switching and optical filtering, and other potential applications. On the other hand, increasing efforts are devoted to the improvement of manufacturing processes for the objectives of proper miniaturization and/or high performance. For example, factors required to be controlled for improved optical performance of electrooptical devices include light directions, light refractive/reflective angles, spectral designs, etc.
Optical filters are key components and offer versatile functionalities in light modulation, optical communication, and wavelength filtering of laser cavities. Different filters exhibit different specific optical functions. Distinguished by functions, optical filters include transmission and reflection filters, bandpass and bandstop filters, narrow bandwidth and wide bandwidth filters. Currently, the most prominent optical filters include thin-film filters and Fabry-Pérot filters. The thin-film filter relying on the interference effect consists of several alternative high- and low-index dielectric thin films coated on a substrate. These stacked films are arranged to be a quarter-wavelength in layer thickness and optimized to eliminate the stress between them. The Fabry-Pérot filter consists of a cavity sandwiched with a specific gap by two parallel partial reflecting mirrors, usually made of metal films or multilayered dielectric films. These filters suffer from various problems such as complex structure with more than 100 individual layers, tight fabrication due to strict tolerance on each layer, and power loss caused by material absorption.
Optical filters based on the guided-mode resonance (GMR) effect have attracted considerable interests because of their simple structures of much fewer layers and superior spectral properties of high efficiency, narrow bandwidth, and angular sensitivity. The GMR filters are commonly composed of dielectric thin-film structures incorporating gratings with periodic modulation of refractive indexes to couple energy diffracted from an incident wave to a leakage mode of the waveguide layer included in the structure.
Please refer to FIG. 1, in which the basic wave-filtering operation of a GMR filter is illustrated. The GMR filter 10 includes a grating structure 11 and a waveguide structure 12, wherein the grating structure 11 is disposed on the waveguide structure 12 with a pattern of lattices and is present with such a small thickness that the operation of the waveguide structure 12 to determine the guided mode of incident light will not be affected by the grating structure 11. When incident light 13 is emitted onto the grating structure 11, the grating structure 11 performs a light-splitting function to have a portion of the incident light 13 coupled to the waveguide structure 12. When the incident light 13 coupled to the waveguide structure 12 is in a phase-matching state admissible by the waveguide structure 12, resonance effect appears. Accordingly, light may traverse the GMR filter 10, as indicated by the direction B, with a selected resonance wavelength after being refracted by the waveguide structure 12. Due to the presence of the grating structure 11, the traversing light is not always kept stable. On the contrary, light reflection by the grating structure 11 would be rendered to be transmitted out of the waveguide structure 12 in the leakage mode. Furthermore, light transmission of a non-resonance portion of the incident light 13 out of the waveguide structure 12 would also be rendered.
It is obvious that GMR filters tend to integrate additional thin-film structures or stack multiple GMR filters to enhance its resonance performance for various applications. Moreover, GMR filters are expected to integrate other passive and active optoelectronic devices to form microsystem chips or become tunable GMR filters. However, most of these devices cannot be fabricated on silicon substrates but on lower-index substrates such as quartz, and then, a thin-film structure of higher refractive index is coated as the guided layer. The utilization of a nonsilicon substrate would restrict the further integration of GMR filters with other optoelectronic devices.